A lithium ion battery is one of secondary batteries with nonaqueous electrolytes. With the lithium ion battery, a lithium metal oxide is used as an active material in a positive electrode, while a carbon material such as graphite is used as an active material in a negative electrode. During charging, lithium ions released from the active material of the positive electrode are absorbed to the active material of the negative electrode. During discharging, the lithium ions stored in the active material of the negative electrode are released and absorbed to the positive electrode. This movement of the lithium ions between the electrodes allows an electric current to flow between the electrodes. That is, with the lithium ion battery, the lithium ions moving between the electrodes are responsible for electric conduction. Since the lithium ion battery has a high energy density, this is used, for example, as a battery for an electric vehicle.
With such a lithium ion battery, lithium ions moving between the electrodes during use are precipitated(deposited) on the surface of the negative electrode to form a film, which is fixed there, with the result that lithium ions movable between the electrodes decrease. Consequently, the capacity of the battery may decline, that is, the battery may degrade.
As a solution to the degradation of the battery, a secondary battery with a nonaqueous electrolyte is known which has a third electrode provided with metallic lithium making no contact with an electrolytic solution (see, for example, Patent Document 1).